Recently, an oxyfuel combustion device has been studied as one of techniques for reducing emission of carbon dioxide (CO2) which is the to be one of reasons for global warming; attention is attracted to, for example, a coal burning boiler for oxyfuel combustion of pulverized coal in which oxygen is used as an oxidizing agent in lieu of air to produce exhaust gas mainly comprising CO2. It has been conceived that such exhaust gas with high CO2 concentration is compressed, cooled and recovered as liquefied carbon dioxide for disposal; alternatively, the exhaust gas may be pressurized and stored in the ground. Such exhaust gas treatment system for a coal burning boiler for oxyfuel combustion is disclosed in Patent Literature 1.
As shown in Patent Literature 1, the exhaust gas from the coal burning boiler for oxyfuel combustion of coal contains not only CO2 but also impurities derived from coal feedstock such as nitrogen oxides (NOx), sulfur oxides (SOx), hydrargyrum (Hg), hydrogen chloride (HCl) and dust. The NOx as impurities may include nitrogen dioxide (NO2) which is contacted with and dissolved in water into nitric acid. However, in the exhaust gas from the coal burning boiler having less oxygen (O2), NOx exists substantially in the form of nitrogen monoxide (NO) which is water-insoluble and unremovable, for example, by spraying; the SOx is contacted with and dissolved in water into sulfuric acid; and the HCl is dissolved in water into hydrochloric acid. Such impurities are to be preferably removed at an early stage especially because the sulfuric acid is known to have a problem of eroding appliances in an exhaust gas treatment system and the Hg is known to damage low-temperature aluminum members in a heat exchanger. There is also a problem that commingling of the impurities lowers a purity of CO2 and makes it difficult to liquefy the same through compression and cooling. Thus, for example in a coal burning boiler for oxyfuel combustion, an exhaust gas treatment system is required for removal of impurities.
Thus, it may be conceived that an exhaust gas treatment system, for example, for a coal burning boiler for oxyfuel combustion is provided with a so-called wet desulfurization device for removal of SOx as shown in FIG. 2 of Patent Literature 1 which is of a spray- or a packed-column type and which has been used for a conventional air burning boiler or the like. NOx derived from coal feedstock is produced in the exhaust gas from, for example, the coal burning boiler for oxyfuel combustion so that a denitration device for removal of NOx is required, though it is not disclosed in Patent Literature 1.
It is known that provision of the wet desulfurization device as mentioned above brings about removal of not only SOx but also dust as well as a certain level of Hg and HCl. When the exhaust gas with the impurities being removed therefrom by the exhaust gas treatment has a high Hg concentration, a Hg-removing column is arranged for removal of Hg by adsorbent or the like.